ABSTRACT
As long as the seminal fluid of a ripe sea-urchin remains undiluted, little or no movement occurs on the part of the spermatozoa. If, however, a small drop of this fluid comes into contact with sea-water, the cells at the surface of the drop at once become intensely active and eventually the whole of the spermatozoa exhibit lively movement. There appear to be at least four possible causes for this immediate activation in sea-water : (i) The failure to move in the testicular fluid may be due to the presence of an inhibitory substance in the medium, so that on dilution this substance is removed and the spermatozoa are activated, (ii) Sea-water may contain some element, absent from the testicular medium, which is necessary for movement. (iii) The viscous resistance of the testicular fluid may be too high for active movement, (iv) Each spermatozoon may exert some form of inhibition on the movements of its neighbours. The following experiments were designed to test these hypotheses.
The mutual restraint exercised by one cell on another may perhaps be termed allelostasis.
